This invention relates to a contact lens and to a method of producing a hard contact lens having permanent surface wettability and good wearability.
To attain better wearability of a contact lens and to decrease a foreign body sensation when the contact lens is put on, it is important to improve compatibility of the lens surface with the cornea. Optimization of the design elements, including size and form of the lens, is of course an important point, but an essential point to obtain a contact lens having good wearability is the chemical affinity of the contact lens surface with the cornea (a living tissue). One example of a surface treatment process for permanently rendering corneaaffinity to a contact lens surface is graft polymerization of a hydrophilic monomer such as acrylamide on the lens surface, thereby improving functions of the lens surface such as wettability (including water retaining capacity) and sliding capacity.
In order to retain wettability for a long time, the inventors developed a method of improving the rubbing resistance of graft polymer chains by cross-linking them with N,N'-methylenebisacrylamide and the like. There are two major steps to carry out the surface graft reaction efficiently. The first step is to generate a radical which becomes a polymerization starting point on the surface of a substrate material. The second step is to remove a polymerization inhibiting factor which hinders graft polymerization reaction (allows growth of the graft chain from the polymerization starting point).
Optimal conditions for preparing a contact lens are not achieved when the steps are carried out independently of each other, but optimal conditions are obtained when the steps are combined. Furthermore, surface graft polymerization reaction conditions differ depending on the substrate materials used and this must be taken into consideration. So far, these two steps have not been combined to achieve optimal results. Described below are current techniques which have been employed for grafting hydrophilic monomers on a contact lens substrate surface.
First, a peroxide is formed on a contact lens surface during discharge treatment. Then a radical is generated by chemically decomposing the peroxide. So far, for the decomposition of the peroxide, pyrolysis has been carried out at a high temperature of 60.degree.-90.degree. C.
The second step, removal of a polymerization inhibiting factor, is as follows. Generally, the most important polymerization inhibiting factor in a solution system is oxygen dissolved in the solution. By removing dissolved oxygen, a graft polymerization reaction can be initiated. So far, in order to remove dissolved oxygen sufficiently, a series of steps is repeated several times: a vessel containing the monomer solution is evacuated, and knocked from outside, then an inactive gas, such as nitrogen, replaces air in the vessel, and the vessel is again evacuated. However, the aforementioned process has serious defects as described below.
First, thermal decomposition of peroxide (radical generation) on the lens substrate surface requires a temperature ranging from 60.degree. to 90.degree. C. The softening point of the currently used lens substrates, which mainly contain an acrylic resin, is within this temperature range, and leads to considerable deformation of the lens substrate and deterioration of optical characteristics (change of lens power, change of transmittance in the visible region and the like).
Regarding the second step, the removal of dissolved oxygen (polymerization inhibiting factor) is preferable for initiating the graft chain growth reaction from the polymerization initiating point and involves a series of steps--evacuating the vessel containing the solution, knocking it from the outside, replacing the gas in the vessel with an inactive gas such as nitrogen, and reevacuating it. These steps are repeated several times and are too complicated to be employed in mass production. Controlling the process is difficult and oxygen dissolved in solution cannot be completely removed and trace amounts remain. Accordingly, the varying trace amounts of dissolved oxygen during polymerization provides fluctuation of graft polymerization conditions, non-homogeneity of graft surface conditions and uncomfortable contact lens wearability in the end.
Methacrylate (which does not include silicon or fluorine), a typical material for a contact lens substrate, has been generally used. A siloxanyl methacrylate (which does not include fluorine) type, a fumarate type and a fluorine containing type contact lens substrates are also known for producing a contact lens having high oxygen permeability. For example, a contact lens substrate containing fluoroalkyl methacrylate as described in Japanese patent application Laid-Open Publication No. 62-294201, is known as a material which shows especially high oxygen permeability. The surface of such a contact lens has high oxygen permeability but low wettability with water; thus the lens feels dry and uncomfortable to a wearer when he or she puts on the contact lens.
Moreover, graft polymerization carried out by contacting hydrophilic monomers alone with the lens surface after the lens surface was treated by discharge under normal pressure, i.e., 1 atm., or reduced pressure, i.e., 10.degree. to about 10.sup.-2 Torr results in weak bonding between the graft polymer and the lens substrate. The graft polymer is relatively easily removed by rubbing with a finger and the like, and wettability returns to the level prior to the graft polymerization.